In a cellular network, such as code division multiple access (CDMA) system, as specified in TIA/EIA/IS-2000, when the user of a subscriber station, e.g., cellular phone, travels from one geographic area to another, the subscriber station must select a communication system upon which to conduct services. The baseline method of selecting a preferred system in a prior art is described in U.S. Pat. No. 6,085,085, as follows: the subscriber station maintains a list of systems, some of which are ‘preferred’ systems, i.e., systems the subscriber station is allowed to use, and some of which are ‘negative’ systems, i.e., systems the subscriber station is not allowed to use. The list of systems is referred to as the preferred roaming list (PRL). Associated with each system in the PRL is a system ID (SID), as well as corresponding acquisition parameters (band, channel, etc.). The PRL is created, loaded and updated by the home service provider of the subscriber station.
The PRL is maintained in such a manner that the subscriber station can readily determine which systems (preferred or negative) cover common geographical regions. The references to common geographic regions refers to areas of common radio coverage. Moreover, the systems that cover a common geographical region are prioritized, i.e., ranked from most desirable to least desirable. The subscriber station's job is to attempt to acquire service on the most available desirable system in the subscriber station's current geographical area. There is no point in trying to acquire service on a system outside of the subscriber station's current geographic region, since systems typically provide service only within a limited geographic region.
The subscriber station does not necessarily know where it is when it powers on. Because of roaming, it could be in a different region than it was when it was last powered down. Therefore, it may not be obvious how to acquire any system, let alone the most desirable system. In the prior art, the subscriber station maintains a list of the most recently used (MRU) systems. As a first measure, attempting to acquire one of these systems makes a lot of sense, because there is a good chance that the subscriber station is currently in a recently visited region.
If the subscriber station cannot acquire a MRU system, it can then try to acquire any preferred system in the PRL, using appropriate acquisition parameters. In the prior art, the subscriber station first attempts acquisition on the ‘easiest’ to acquire systems, e.g., advanced mobile phone system (AMPS), assuming there are any AMPSs in the universal system table. If the subscriber station is unable to acquire one of these systems, then the subscriber station attempts to acquire a system which is ‘most representative’ of systems in the PRL. In other words, the PRL will probably contain numerous systems which differ only with respect to their SIDs and their geographical regions, but which have identical acquisition parameters, e.g., channel number or frequency block designator.
Once the subscriber station acquires a system, the subscriber station can pick up the acquired system's SID from an overhead message. The subscriber station uses the received SID and the acquisition parameters to determine the geographic region in which it is located. The subscriber station can obtain this geographic information from the PRL, whether the acquired system is preferred or negative.
If the system is a preferred PRL system, and is one of the most desirable systems within its geographical region, the subscriber station provides service using that system. If the system is a PRL system but is not one of the most desirable systems within its geographical region, the subscriber station attempts to acquire a most desirable system in that geographical area, by sequentially making acquisition attempts on systems in the geographic region from most desirable to least desirable for the region.
If the system is not in the PRL, the subscriber station of the prior art attempts acquisition on other systems. The subscriber station may return to this system if the subscriber station fails to acquire a preferred system.
The prior art identified above requires that, if the acquired system is not listed in PRL, the subscriber station attempt to acquire other systems. However, it is quite possible that the subscriber station encounters a number of unknown systems before it is able to determine the geographic region. This case is more likely when roaming to a different geographic area, where the acquisition parameters of the PRL systems available in the roaming region may be quite different from the ones for the MRU systems.
For example, consider the case where a subscriber station, having the PRL table shown in Table 1, travels from Region 1 to Region 3. Table 2 defines the available systems that the subscriber station would encounter in Region 1, as well as in Region 3. When the subscriber station was initially located in Region 1, its MRU table would be as shown in Table 3.
TABLE 1PRLGeographicSIDRegionPreferred/NegativeBandChannel(s)30Region 1PreferredCellular384, 7774117Region 1PreferredPCS525, 7504119Region 1PreferredPCS425, 45025Region 1NegativeCellular283, 69150Region 2PreferredCellular384, 7772113Region 2PreferredPCS500, 550, 7252155Region 2PreferredPCS475, 6253Region 3PreferredCellular283, 6914129Region 3PreferredPCS600, 6504157Region 3PreferredPCS600, 675
TABLE 2Available systems in geographic regionsGeographic RegionSIDBandChannelRegion 130Cellular3844119PCS425Region 32Cellular3844157PCS675
TABLE 3MRU table while in Region 1 for PRL SystemSIDBandChannel30Cellular3844119PCS425